Merge r252337 from head:

  Don't try to re-insert an already present but invalid page.

  This could happen if a thread doing a page-in loses a ZFS range lock
  race to a thread writing to the same range

  This fixes "panic: vm_page_alloc: pindex already allocated" in
  http://docs.FreeBSD.org/cgi/mid.cgi?1372165971.96049.42.camel

Submitted by:	avg
MFC r226454 (by bz):

Fix VNET build.